Inkjet printing is a nonimpact process of printing text or images by depositing inkjet ink on a print medium. The inkjet ink includes an ink vehicle and a colorant, such as a dye or a pigment. Pigment-based inkjet inks produce printed images that have good waterfastness and lightfastness in comparison to images produced with dye-based inkjet inks. In addition, pigment-based inkjet inks are formulated to have fast drytimes and to provide good durability to the printed image. While pigment-based inkjet inks have these desirable properties, the pigments are typically insoluble in the aqueous-based ink vehicles. Therefore, the pigments must be stably dispersed to prevent agglomeration or settling out of the ink vehicle.
To form stable pigment dispersions, dispersants or surfactants are added to the ink vehicle. The dispersant is typically a polymer that binds to a surface of the pigment, providing stability to the pigment dispersion. When printing is desired, the pigment is precipitated from the inkjet ink and onto the print medium by solvent evaporation. Alternatively, the pigment is stably dispersed by covalently attaching solubilizing groups, such as polymeric, ionic, or organic groups, to the surface of the pigment. These pigments are referred to in the art as self-dispersed pigments or surface-modified pigments. One category of self-dispersed pigments includes polymer-attached pigments. As used herein, the term “polymer-attached pigment” refers to a pigment having at least one polymer covalently attached to the surface of the pigment.
Inkjet inks that include polymer-attached pigments have numerous advantages, such as fast drytimes, which are typically less than 10 seconds. In addition, when the inkjet inks are printed on a porous paper, the resulting images have good durability (typically less than 100 milli-Optical Density (“mOD”) as measured by a 2-Pass Alkaline highlighter smear test), which is due to large amounts of binder and surfactant present in the inkjet inks. However, the printed images also exhibit poor mottle. Mottling is an uneven print density that results in a splotchy appearance of light and dark areas. Mottling commonly occurs in solid area fills, such as in graphics, when black or cyan inkjet inks are used. A visible mottle rating for these inkjet inks is usually 6 or less, out of a scale from 1 (worst) to 10 (best). These inkjet inks typically include greater than 20% of a binder, greater than 0.1% of a surfactant, and greater than 20% organic solvents. When the inkjet inks having polymer-attached pigments are printed with a cationic fixing system, which includes a cationic polymer and an acidic crashing agent, coalescence effects occur due to the large amounts of binder and surfactant that are present. The cationic fixing system is used in combination with the inkjet inks to improve the durability and image quality of the printed image. The coalescence effects produce a mottled appearance in a complete blackout print area on the porous paper.
It would be desirable to produce a pigment-based inkjet ink that provides improved mottle on porous paper and does not produce coalescence effects with a cationic fixing system.